Mr Jennings, a mechanical fitter, was changing a drain valve in am 11ft (3.4m) deep well at the tank when sparks from his grinding tool reacted with oxygen which had leaked into the tank. He later died in hospital.

The near-miss incident in September happened near to where 55-year-old Mr Jennings was working.

The inquest heard that workers were trying to remove pumps when the alarm on their gas monitors sounded and they were forced to ventilate it until it was safe to enter.

Image caption
The inquest at Selby heard that Mr Jennings was working in an 11ft (3.4m) deep well at the tank

Steve Smith, who worked as Mr Jennings' team leader, said he had been made aware of the previous incident but that all procedures had been correctly followed.

He said the conclusion of a report into the incident at the time was that no further action was required.

"The air was made safe and the work continued safely. The reactor was ventilated as part of the ongoing work for several hours and it was recorded that there were no residual issues," he told the inquest.

Asked whether he highlighted the possibility of oxygen enrichment at the site before Mr Jennings entered, he added: "I can't recall."